Monitoring of recent crustal movements around Cairo by repeated gravity and geodetic observations

Abstract

The mass distribution and density inhomogeneities are one of the main factors
affecting Earth’s dynamics. The temporal gravity variations were used to understand the
surface tectonics and geodynamic modeling after the occurrence of an earthquake of 1992.
The gravity field changes have been used for monitoring recent crustal movements in the
area around Cairo and Southern part of Delta, Egypt. More than five successive gravity
measurement campaigns were performed in parallel with the geodetic technique (GPS).
The gravity changes were determined and correlated with seismic activity for all periods
of observations. It was found that there is a certain relation shown by increasing of
gravity values before the main seismic activity. As an example, a relatively considerable
increase of gravity values was noticed for the network between the epochs of 2000 and
2004. Otherwise, the temporal gravity variations exhibited a considerable decrease in
gravity values between the two campaigns of 2004 and 2007 for the same stations. This
behavior could be explained by compressive deformation and strain buildup stage before
the Southwestern Cairo earthquake (July 31, 2005 with magnitude of 4.3) and the stress
release stage occurred after the main-shock. In addition, the results of geodetic measurements
of the network around Cairo after five campaigns showed that the estimated
horizontal velocities for almost of points are 5.5 mm/year in approximately NW direction.